The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Referring now to FIG. 1, microelectromechanical system (MEMS) devices 10 may be used to adjust the focus of lens arrays 12 in cameras 14. For example, the lens array 12 may include fixed lenses 22 and 24 and a moveable lens 26 arranged therebetween. A position of the moveable lens 26 may be adjusted by an output voltage of a voltage source VS.
While the voltage to displacement of the MEMS device 10 is generally linear, it can be affected by manufacturing variations. Therefore, the amount of voltage corresponding to a predetermined lens position may vary from one camera to another. Capacitance measurement may be used after manufacturing to characterize the response of the MEMS device 10. This kind of measurement typically employs complex electrical measuring devices and other external components.
Referring now to FIG. 2, while manufacturing variations may be accommodated by precise measurement before shipment, other variations cannot. For example, the voltage to displacement relationship can also be affected by gravity (which changes with the orientation). Thus, the voltage to displacement relationship depends upon whether the camera is facing up, down or horizontal. The voltage to displacement relationship also varies with temperature and hysteresis. All of these variations tend to limit the use of the MEMS device 10 in autofocus applications.
Optical measurement systems can be used to measure displacement. However, optical measurement systems are typically difficult to integrate with portable compact cameras. Some conventional capacitance measuring devices apply an AC voltage signal to one electrode of a device and probe the voltage (or current signal) on a second electrode of the device using a known series resistor. The capacitance information can be determined from the amplitude ratio Vrms.